Growth and nitrogen metabolism in Sophora japonica (L.) as affected by salinity under different nitrogen forms.

Sophora japonica is a leguminous tree species native to China. To explore the nitrogen (N) source preference and its impact on stress tolerance, a hydroponic experiment was designed in which S. japonica seedlings were supplied with sole ammonium (NH4+) or sole nitrate (NO3-) nutrition under 75 mM NaCl-induced salt stress. The growth and N metabolism performance were investigated. In the absence of NaCl, plants fed NH4+ showed better root growth than those fed NO3-, but there was no difference in aerial part growth. Salinity inhibited the root growth of NH4+-fed plants and the shoot growth of NO3--fed plants, while the total N accumulation was suppressed under either N form. Specifically, in NH4+-fed plants, salinity significantly increased the net photosynthetic rate, root NH4+ content and root antioxidant enzyme activities. Higher nitrate reductase (NR) activities but lower glutamate synthetase (GS) activities were observed in both leaves and roots. Leaf AMT1.1 and AMT2.1a in NH4+-fed plants positively reacted to salt stress, whereas the expression of four AMTs was reduced or remained unchanged in roots. In contrast, salinity suppressed the net photosynthetic rate, antioxidant enzyme activities, and GS activity in the leaves of NO3--fed plants. Upregulation of NPF1.2, NPF2.11, NPF4.6 and NPF7.3, as well as unaltered NR activity, caused higher NO3- content in the leaves. Moreover, NR and glutamate synthase (GOGAT) activities together with the transcription of most NRTs were promoted by salinity in the roots of NO3--fed plants. Additionally, compared to those treated with NH4+, in response to salinity, NO3--treated seedlings showed more intensive repression of the net photosynthetic rate, chlorophyll content, and both shoot and root growth. Overall, these results suggest that S. japonica plants grew better in NH4+ medium than in NO3- medium, and the different N metabolism responses improved S. japonica tolerance to salinity with NH4+ application. This study provides new insights for understanding the mechanism of salt tolerance, breeding resistant varieties of S. japonica, and developing scientific fertilization management strategies during the seedling cultivation period.